1. Field of the Invention
The present invention relates generally to a storage device and its seek control method providing a seek control for moving a head in a disk radial direction by drive of an actuator to position the head at a target track, and more particularly to a storage device and its seek control method improving the seek performance through a reduction of the seek time while keeping the stability of lead-in to a target track in the short distance seek not exceeding several tens of tracks.
2. Description of the Related Arts
In conventional information storage devices, in particular removable disk units represented by optical disk units, disks are removed or mounted for use with a need to provide a stable seek control against various disturbances proper thereto. For example, fixed disk units such as hard disk units are rarely affected by disturbances in the track radial direction arising from the disk eccentricity, whereas the optical disk units, e.g., a 3.5 inch 1.3 GB magneto-optical disk may suffer from a radial disturbance as much as 50 .mu.m relative to 0.9 .mu.m track pitch. In such disturbance conditions, a stable migration from the target track seek control to the track following control may remarkably be impaired, which may lead to frequent retries in the track following control and thus to a heavy degradation of the drive performances. To cope with this, the seek control upon the migration to the track following control provides a velocity control so as to allow the radial relative velocity with respect to the target track to be a desired value. In a typical velocity control, the target velocity corresponding to the number of remaining tracks to the target track is acquired by derivation from a previously provided table or from calculating expressions.
FIG. 1 is a block diagram of the conventional velocity control. A tracking error signal from a tracking error signal detection circuit 320 is converted into a TES zero-cross signal TZCS by a zero-cross signal detection circuit (TZC circuit) 300. A position/velocity detector 302 acquires a position signal and a relative velocity signal in the disk radial direction of an objective lens mounted on the head moving mechanism of the actuator. From this position signal, a target velocity generator 304 issues a target velocity signal. An addition unit 306 finds a difference between the relative velocity signal and the target velocity signal to issue a velocity error signal, which in turn is fed to a phase compensator 308 for proportional gain or phase compensation to obtain a velocity feedback signal. The position signal is fed to a target acceleration generator 312 to obtain a target acceleration signal at the same time. The target acceleration signal results in an acceleration feedforward signal serving as an acceleration signal for moving the actuator to the target track. The velocity feedback signal and the acceleration feedforward signal are added together in an addition unit 314 at the output stage, the added signal resulting via a driver 316 in a seek control signal for driving a head moving mechanism 318. Upon the acceleration, a switch 310 may be opened so as to permit the output of only the acceleration feedforward signal without any output of the velocity feedback signal.
In such a conventional seek control, however, the signal quality of the TES zero-cross signal TZCS may possibly induce any degradation of both the position signal and the relative velocity signal, with the result that the velocity feedback signal may become noisy. This deficiency remarkably appears in the low-velocity region immediately before the migration to the track lead-in. In the event of occurrence of hunting where the actuator velocity may vary to a great extent by noises, the target track may be reached previous to the recovery of the hunting. Accordingly, in the case of seek control in the low-velocity region immediately before lead-in to the track or of low-velocity seek control as in the short distance seek crossing a relatively short track interval, the influences of the noises need to be minimized. In order to solve such a problem, the present inventors conceived a way of generating a target velocity function or a target velocity relative to the elapsed time from the start of seek control, instead of the conventional generation of the target velocity or the target acceleration relative to the position. This seek velocity control system generating a target velocity through the input of the elapsed time is free from any influences by the position detection errors or detection noises, with the result that the effects of the noises on the velocity feedback signals can be reduced to a minimum.
FIGS. 2A to 2C are time charts of the short distance seek control system using the method conceived by the present inventors. Herein, with respect to the time t on the axis of abscissas, FIG. 2A depicts the velocity, FIG. 2B depicts the acceleration and FIG. 3C depicts the position. The seek control section is divided into three segments, i.e., an acceleration control segment, a deceleration control segment and a constant-velocity control segment such that the respective control segments are changed over depending on the elapsed time from the start of seek. In the acceleration control segment immediately after the start of seek, the actuator is subjected to an acceleration control at a certain acceleration A.sub.0 for a predetermined time T.sub.0 or for a predetermined distance X.sub.0 so that the relative velocity V.sub.D upon the termination of acceleration is measured. In the next deceleration control segment, the decelerated acceleration control and the velocity control are carried out at one time. From the detected velocity V.sub.D upon the changeover to the deceleration, the decelerated acceleration control figures out a target decelerated acceleration trajectory A(t) of FIG. 2B for deceleration to a predetermined velocity V.sub.C allowing a changeover to the track following control for the target track in a predetermined time T.sub.1 using ##EQU1##
This decelerated acceleration trajectory A(t) is a function achieving an acceleration A.sub.1 at the start of deceleration and acceleration zero after the elapse of time T.sub.1. In this case, the acceleration A.sub.1 at the start of deceleration is derived from ##EQU2##
A target velocity trajectory V(t) is derived on the basis of the decelerated acceleration A.sub.1 of the expression (1) from the following expression, to represent the trajectory at the time T.sub.1 of FIG. 2B. The target velocity V(t) at that time is represented by the time function ##EQU3##
achieving the velocity V.sub.C after the elapse of time T.sub.1. Afterward, a constant-velocity control is provided at the target velocity V.sub.C and, when reaching the vicinity of the target track, a migration is carried out to the track following control. The migration to the track following control is effected for example by providing a seek control till the track precedent one track to the target track, whereat a deceleration pulse is issued to allow a movement to a region capable of follow-up on the target track previous to the migration to the track following control. Generation of such a target trajectory reducing the target velocity and the target decelerated acceleration depending on the elapsed time is advantageous in lessening the influences of variances of viscosity resistance on the actuator moving mechanism relying on the velocity variations or in suppressing the excited vibrations of the mechanism arising from the rapid change of acceleration.
In the event of the control generating the target trajectory based on the elapsed time, however, a longer seek distance to the target track may cause an extension of only the time of movement to the target track by the constant-velocity control at the constant velocity V.sub.C after the termination of deceleration, that is, only the time T.sub.C1 of FIG. 2B, resulting in an extended seek time proportional to the distance to the target track. Referring to FIGS. 1 and 2A to 2C, this problem is described with a seek controller by way of example employed in the optical disk unit making access to the optical disk. Although the optical disk can be for example a phase change (PD) type optical disk or magneto-optical (MO) disk, the optical disk unit making access to the magneto-optical disk is typically contemplated herein. In the acceleration control segment immediately after the start of the seek control of FIGS. 2A to 2C, an acceleration control is provided. This acceleration control provides a predetermined acceleration A.sub.0 during the time T.sub.0. As a result, the velocity reaches V.sub.D and the position reaches X.sub.0. For that duration, the switch 310 is opened to shut out the velocity feedback signal. In the deceleration control segment which follows, an initial value A.sub.1 of the target acceleration is derived from the expression (2) so as to achieve the velocity V.sub.C after the time T.sub.1 on the basis of the velocity V.sub.D detected upon the changeover, and then a target acceleration signal A(t) and a target velocity signal V(t) corresponding to the elapsed time are derived from the expressions (1) and (3), respectively. At that time, the switch 310 is closed to provide a velocity feedback control for the elapsed time of the decelerated acceleration control. After the elapse of the time T.sub.1, the velocity V(t) reaches a velocity V.sub.C, with the acceleration of zero and the position X.sub.1. Herein, the velocity V.sub.C is enough a low velocity to allow a migration to the track following control, and the velocity feedback control allows a selection of enough a feasible velocity. For example, in case of a 3.5 inch 1.3 GB magneto-optical disk of 0.9 .mu.m track pitch, V.sub.C =7 mm/s would ensure a velocity control band of the order of 500 Hz to 1 KHz since the TES zero-cross signal TZCS has a frequency of 7.8 KHz which is used as a sampling frequency of the seek control system. After the elapse of time T.sub.1, the actuator is velocity controlled at the constant target velocity V.sub.C, with the position being increased from X.sub.1 linearly with respect to the time. Assume now that the position upon the migration to the track following control is for example X.sub.2 which is the position precedent one track to the target track. Then, the instant that detection has been made of the arrival of the actuator at the position X.sub.2 after the elapse of the time T.sub.C1, a migration is carried out to the track following control. The time T.sub.E upon the migration to the track following control is given by EQU T.sub.E =T.sub.0 +T.sub.1 +T.sub.C1 (4)
A track lead-in time not shown is added to this period of time till the time T.sub.E to obtain a total seek time. In case of such a provision of the seek control generating the decelerated acceleration and decelerated velocity target trajectories based on the elapsed time, the time (T.sub.0 +T.sub.1) till the termination of the deceleration control is unvarying irrespective of the increase in the distance to the target track, but the time T.sub.C1 of the constant-velocity control will vary depending on the seek distance. Thus, accordingly as the seek distance increases, the time T.sub.C1 of the constant-velocity control will also be increased, resulting in a longer time taken to reach the target track in proportion to the seek distance. To attenuate this drawback, the target velocity V.sub.C in the constant-velocity control may be increased. However, too a high velocity may possibly impair the stability of migration to the track following control.